Persons who must spend long periods of time in one position, whether sifting or lying, often experience tissue injury and discomfort because the interaction between the supporting structure and the area of the person being supported often produces pressure sores and related conditions. Persons in wheelchairs are especially susceptible to the formation of pressure sores and the related tissue injury and discomfort they cause. Typically, persons who use wheelchairs have more of the factors that are considered to promote formation of pressure sores, e.g. age, activity levels (either very active or inactive), general health, weight, etc. Therefore, a need exists to modify the physical attributes of the supporting device that cause pressure sores to form in the first place.
To better understand the need for the present invention, an analysis of the likely causes of tissue injury is necessary. Pressure sores and related degenerative conditions result from an ulceration of the skin and/or deeper tissue due to unrelieved pressure, shear forces, and/or frictional forces. This condition occurs most frequently in persons confined to a bed or a wheelchair for long periods of time. The onset of these ulcers is believed to be triggered by a hypoxia condition--the decrease flow of or lack of oxygen to the subject tissue. As a consequence of this diminished oxygen supply to hard and soft tissue sites, aerobic and/or anaerobic microorganisms and their waste products can accumulate in these areas and cause infection and bacteremia leading to increased tissue breakdown and decreased healing abilities. Research has shown that healing wounds had absolutely no anaerobic bacteria and that few colonies and types of aerobic bacteria were present. Non-healing wounds, however, had very high counts of both aerobic and anaerobic bacteria. Consequently, hypoxia of tissue subject to pressure not only is the likely cause of pressure sores, but also interferes with the natural healing process.
Because the number of persons who lack full mobility has increased as the median age of the population has increased, there is a greater number of mobility impaired persons using wheelchairs or spending considerable time in bed. Until recently, little attention had been given to modifying the supporting devices used by these people. Now, the combination of a greater segment of the population using or confined to these devices, and an increased understanding of the causes and effects of pressure sores, has created a need for products to make these supporting devices more comfortable and therapeutic. Ideally, these products alleviate pressure sore formation, increase user comfort, and enhance body support and position.
In the art to which this invention is directed, comfort is associated with reduction of pressure in critical areas. The less pressure in critical areas, the greater the comfort and the fewer number of pressure sores. Because pressure occurring at any given point on a person's body is a result of the force, i.e. the weight acting on the person per unit area affected, the goal is to increase the area subject to this force, thereby decreasing effective localized pressure. Therefore, the ideal cushion would have a custom base molded to the user to maximize comfort while enhancing support. Ideally, the user would never shift his or her position nor change his or her physical attributes. Of course, such conditions and restraints are not practical. Hence, a cushion must be adaptable to various sitting positions that might occur through normal use and weight shifting, and be adaptable to changes in the person's physical attributes. To meet these needs, a variety of seat cushions have been proposed and used.
In the field of wheelchair cushions, four types of cushions predominate: foam devices, viscoelastic foam devices, gel devices, and fluid flotation devices. Research has shown that in addition to the discomfort and health risk associated with pressure sores, a person's comfort when using a cushion type device is also affected by poor distribution of stresses, moisture accumulation, heat transfer (either excess accumulation or loss), and stability. Research has also shown that the efficacy of the cushion (i.e. its support) includes such parameters as stability, weight of the cushion, frictional properties, thickness of the cushion, cost, and durability. Each type of cushion has its advantages and disadvantages. Some cushions distribute pressure very well but do so at the cost of excess heat transfer, moisture accumulation, or weight. Other cushions provide low humidities due to their porous properties but do not allow heat to flow freely from a person's skin, thus increasing perspiration and decreasing comfort. Still other cushions are light and easy to transport but do not offer an effective support in areas.
While each type of cushion has successfully been used to mitigate specific instances of the formation of pressure sores, recent advances in foam technology has made foam type cushion, a cost verses performance leader. When discussing foam type cushions, two measurements are primarily used: Indentation Force Deflection (IFD) and foam modulus. IFD values are measured by taking a 15".times.15".times.4" foam sample and measuring the force needed to cause a 25% reduction in foam thickness by depressing an eight inch diameter disk therein. For example, an IFD value of 40 pounds means that a force of 40 pounds is required to depress the eight inch diameter disk, having an area of approximately 50 in.sup.2, one inch into the foam sample. Modulus is defined as the IFD at 65% of the sample thickness divided by the IFD at 25% of the sample thickness. These two methods for determining the characteristics of a foam sample provide the best measure for determining what type of foam should be used for a particular application.
Traditionally, foam type cushions of the prior art comprised a single section of foam. The foam may or may not have been contoured and may or may not have had a cover. More recent cushions have incorporated multiple sections of foam. These cushions essentially stacked sections of foam, with or without inserts, upon one another to achieve a cushion having varying properties.